Metallocyclic Ni₄Ln₂M₂ single-molecule magnets

We herein report the synthesis, crystal structure and magnetic properties of nine new heterotrimetallic complexes. [Ni(Me₂valpn)] (H₂Me₂valpn = N,N'-bis(3-methoxysalicylidene)-2,2-dimethyl-1,3-diaminopropane) was used as the precursor to construct phenoxo-bridged [Ni₂Ln]³⁺ (Ln³⁺ = Dy, Tb, Gd and Y) species that were respectively connected by two [M(CN)₆]^3- (M = Cr, Fe or Co) anions to form octanuclear cyclic complexes, i.e. {[Ni(Me₂valpn)]₂Ln(H₂O)M(CN)₆}₂ in which Ln = Y, Gd, Tb and Dy and M = Cr, Fe or Co (1-9). Each of the complexes contains many lattice-bound molecules of solvation. The Ni(ii) ions are penta-coordinate, while the Ln(iii) ions are nine coordinated with a muffin geometry. The fitting to the χ_mT vs. T curves of complexes 6-9 gave the parameters of J_NiCr = 11.82 cm^-1, J_NiGd = 0.94 cm^-1 and g = 2.04 for complex 6, J_NiFe = 10.58 cm^-1, J_NiGd = 1.24 cm^-1 and g = 2.03 for complex 7, J_NiCr = 9.4(1) cm^-1, zJ' = -0.050(2) cm^-1 and g = 2.06(1) for complex 8 and J_NiFe = 4.9(7) cm^-1, zJ' = -0.35(2) cm^-1 and g = 2.24(1) for complex 9, respectively. The dynamic magnetic investigations demonstrate that complexes 1-5 display single-molecule magnet properties with an effective energy barrier (U_eff) of 38.9 K (1, M1 = Dy, M2 = Cr), 37.2 K (2, M1 = Dy, M2 = Cr), 24.4 K (3, M1 = Dy, M2 = Co), 21.9 K (4, M1 = Tb, M2 = Cr) and 29.6 K (5, M1 = Tb, M2 = Fe), respectively. Complex 1 shows the highest energy barrier among the octanuclear [Ni₄LnM₂] (Ln = Dy or Tb, M = Fe, Cr, Co or W) system. Although the [Ni₄Dy₂Cr₂] complexes have U_eff higher than that of [Ni₄Dy₂Fe₂], complex [Ni₄TbCr₂] shows lower U_eff than that of [Ni₄TbFe₂]. The results indicate that besides the M-C[triple bond, length as m-dash]N-Ni magnetic coupling the lanthanide ions can significantly affect the magnetic performances of heterotrimetallic SMMs as well. Moreover, the SMMs are achieved when diamagnetic Co(iii) was substituted by paramagnetic Cr(iii) or Fe(iii) in the [Ni₄Tb₂ M₂] system, suggesting that the trimetallic strategy is effective in the construction of new 3d-4f SMMs.